Neisseria meningitidis (Nm) is one of the major causes of meningitis and bacteremia world-wide. The incidence of Nm disease is relatively rare in developed countries despite relatively common nasal pharyngeal carriage. The reasons why some strains cause disease but others do not are unknown. Currently, there is no vaccine that can prevent disease caused by all Nm strains, particularly capsular group B. We have discovered that the product of Orf6, one of nine genes found in prophage DNA that has been linked to invasive disease caused by Nm is an immunoglobulin (Ig) binding protein (Igbp). Orf6 is homologous to TspB, a T-cell and B-cell stimulating protein identified earlier by Ala'Aldeen and coworkers (1-4). Importantly, we also found that, TspB/Orf6 (hereafter TspB) is only expressed on the surface of Nm cells when the bacteria are cultured in the presence of human serum. Thus, functionally active TspB is not expressed when bacteria are cultured in commonly used chemically defined media or microbiological media that contains supplements derived from animal sources. The latter suppresses expression even when human serum is present. TspB appears to be specific for the Fc portion of human Ig and may preferentially recognize glycans modifying the Fc domain. TspB binds to mouse Ig but with lower affinity. However, mouse serum does not stimulate expression of Ig binding activity in Neisseria. Human Ig binding by TspB results in activation of human complement but does not lead to productive bacteriolysis. TspB may function by subverting the mechanism of human complement activation to prevent bacteriolysis. These discoveries provide possible explanations for why the prophage is associated with invasive meningococcal isolates, why a function for Orf6 was not apparent in knock out mutants, and why TspB stimulates T and B cells. Knowing the function of TspB makes it a particularly attractive vaccine candidate since antibodies that block the function of the protein inactivate what may be an important mechanism for survival in human blood while at the same time high affinity anti-TspB antibodies may mediate complement dependent bacteriolysis. In Aim 1 we propose to characterize the expression of TspB by Nm strains and identifying molecules in human serum that stimulate surface expression. In Aim 2 we will produce recombinant TspB and subdomains for use in characterizing the structure and function of TspB and as vaccine antigens. In Aim 3 we will evaluate the immunogenicity of TspB derivatives in mice and measure the functional activity of anti-TspB derivative antibodies with respect to bacterial binding, mediating serum bactericidal activity and protection in an ex vivo human plasma model of meningococcal bacteremia. The results of the proposed studies will lead to a greater understanding of the role of TspB in invasive meningococcal disease and possibly a new vaccine candidate for preventing meningococcal disease, particularly by NmB strains for which there is no broadly protective vaccine. PUBLIC HEALTH RELEVANCE: The proposed project will be to characterize expression of a gene that has been linked to invasive meningococcal disease and to determine whether the protein encoded by the gene is useful as a vaccine to provide broad protection against disease caused by Neisseria meningitidis bacteria.